1. Field of Invention
The invention relates to an overvoltage protection element with a housing, with at least one overvoltage-limiting component which is located in the housing, especially a varistor, with at least two connecting elements for electrical connection of the overvoltage protection element to the current path or signal path to be protected, and with an electrically conducting disconnection element which in the normal state of the overvoltage protection element by its first end is in electrically conductive contact with the first connecting element and by its second end to the overvoltage-limiting component, the second end of the disconnection element being connected via a solder site to the overvoltage-limiting component and the solder connection which is implemented at the solder site between the overvoltage-limiting component and the second end of the disconnection element being separated when the temperature of the overvoltage-limiting component exceeds a given response value so that the disconnection point formed in this way when the overvoltage-limiting element is thermally overloaded electrically isolates it.
2. Description of Related Art
The initially described overvoltage protection element with a thermal disconnector is already known from German Patent DE 42 41 311 C2. In this overvoltage protection element, the first connecting element is connected via a flexible conductor to a rigid disconnection element whose end facing away from the flexible conductor is connected via a solder point to a terminal lug provided on a varistor. The other connecting element is connected directly to the varistor via a flexible conductor. The disconnection element is exposed to a force from a spring system which leads to the disconnection element moving away from the terminal lug when the solder connection is separated so that the varistor is electrically isolated in a thermal overload. By way of the spring system, when the solder connection is separated, a telecommunications contact is activated at the same time so that remote monitoring of the state of the overvoltage protection element is possible.
German Utility Model DE 20 2004 227 U1 and corresponding U.S. Pat. No. 7,411,769 disclose an overvoltage protection element in which the state of a varistor is monitored according to the principle of a temperature switch so that, when the varistor is overheated, a solder connection is separated which is provided between the varistor and a disconnection element; leading to electrical isolation of the varistor. Moreover, when the solder connection is separated, a plastic element is pushed by the reset force of a spring out of a first position into a second position in which the disconnection element, which is made as an elastic metal tongue, is thermally and electrically isolated from the varistor by the plastic element. Since the plastic element has two colored markings located next to one another, it acts additionally also as an optical state display, by which the state of the overvoltage protection element can be easily read off directly on site.
European Patent EP 0 716 493 B1 discloses an overvoltage protection element with two varistors, and two disconnection means which can individually isolate a respective one of the varistors on their live end. The disconnection means each have an elastic disconnection tongue, the first end of the disconnection tongue being permanently connected to the first terminal and the second end of the disconnection tongue, in the normal state of the overvoltage protection element, being attached to a connecting tongue on the varistor by way of a solder site. If impermissible heating of the varistor occurs, this leads to melting of the solder connection. Since the disconnection tongue in the soldered-on state (normal state of the overvoltage protection element) is deflected out of its rest position and is thus pretensioned, the free end of the disconnection tongue moves away from the connecting tongue of the varistor when the solder connection softens, by which the varistor is electrically isolated.
An overvoltage protection element with a thermal isolating mechanism is also known from European Patent EP 0 987 803 B1. In this overvoltage protection element, one end of a rigid, spring-loaded slide, in the normal state of the overvoltage protection element, is soldered both to the first connecting element and also to the terminal lug connected to the varistor. Impermissible heating of the varistor, here, also leads to heating of the solder site so that the is pulled out of the connecting point between the first terminal and the terminal lug slide as a result of the force of a spring acting on it; leading to isolation of the varistor.
The known overvoltage protection elements are generally made as “protective plugs” which, together with the bottom part of the device, form an overvoltage protection device. For installation of such an overvoltage protection device which, for example, is designed to protect the phase-routing conductors L1, L2, L3 and the neutral conductor N, and optionally, also the ground conductor PE, in the known overvoltage protection devices, there are the corresponding terminals for the individual conductors on the bottom part of the device. For simple mechanical and electrical contact-making of the lower part of the device to the respective overvoltage protection element, in the overvoltage protection element, the connecting elements are made as plug pins for which there are corresponding sockets which are connected to the terminals in the lower part of the device so that the overvoltage protection element can be easily plugged onto the bottom part of the device.
In these overvoltage protection devices, installation and mounting can be carried out very easily and in a time-saving manner due to the capacity of the overvoltage protection elements to be plugged in. In addition, these overvoltage protection devices in part still have a changeover contact as the signaller for remote reporting of the state of at least one overvoltage protection element and an optical state display in the individual overvoltage protection elements. It is indicated by way of the state display whether the overvoltage-limiting component, which is located in the overvoltage protection element, is still serviceable or not. The overvoltage-limiting component is especially varistors, here, but depending on the application of the overvoltage protection element, also gas-filled surge arresters, spark gaps or diodes can be used.
The above described thermal isolation device which is used in the known overvoltage protection elements and which is based on melting of a solder connection must perform several functions. In the normal state of the overvoltage protection element, i.e., in the state in which it is not disconnected, a reliable and good electrical connection between the first connecting element and the overvoltage-limiting component must be ensured.
In this case, the disconnection point must satisfy, especially, the requirements of short-circuit strength and pulse current strength. This dictates a solid execution of the current-carrying parts, i.e., especially of the disconnecting element and a low-resistance and mechanically stable connection between the elements of the disconnection point. Moreover, when a certain threshold temperature is exceeded, the disconnection point must ensure reliable isolation of the overvoltage-limiting component and continuous puncture strength and resistance to creepage.
In the known overvoltage protection elements which have a thermally separating disconnection point, the problem exists that, during the thermal separation, a fault current flows by way of the component and leads to heating of the component to be isolated. In this way, when the disconnection point opens, an arc can form by which the vicinity of the disconnection point is thermally loaded. Moreover, in the vicinity of the disconnection point, the metal vapor from the arc precipitates. These loads in the vicinity of the disconnection point lead to a reduction of the dielectric strength in the region of the disconnection point so that the required puncture strength and resistance to creepage cannot always be ensured. This problem is then further exacerbated when the overvoltage protection element is to have dimensions as small as possible so that, after the disconnection point is separated, only a relatively short distance can be achieved between the second end of the disconnection element and the overvoltage-limiting component or the terminal lug.